The present invention relates to a drafting film which includes a plastic base which is provided on at least one surface with an adhesive layer and with a pigmented coating system which comprises a homopolymer or copolymer of vinyl acetate or a partially hydrolyzed polyvinyl acetate, hydrophobic polymeric compounds, and delusterants or pigments. Also, the invention relates to the use of the drafting film in preparing a diazo sensitized material.
A drafting film is, as a rule, made from a cellulose acetate, polyvinyl chloride, polypropylene or, preferably, polyethylene terphthalate film which has an adhesive layer on at least one surface. The film is coated with a pigmented coating system, to give a surface which can be marked or written on with pencils and inks. The pigmented coating system consists either of a pigmented layer to which even lines of aqueous drawing inks adhere or a separate top layer is applied to the pigmented layer in order to make India ink adhere.
There is described in German Offenlegungsschrift No. 2,417,879 a film material which can be marked and which contains a top layer applied to a pigmented layer, which consists of a homopolymer of vinyl acetate, a polymeric compound which mainly contains hydroxyl- or carboxyl-containing monomers, and a hydrophobic agent. Such a top layer has, in principle, proven water- or moisture-sensitive. If the hydrophobic agent content is high, wetting with aqueous drawing inks is also unsatisfactory. Finally, this film material lacks an antistatic finish.
Further, German Offenlegungsschrift No. 2,347,324 describes a top layer for a matt film material which can be marked and which contains an external layer as an antistatic coating on a polyester carrier which is provided with a layer of film-forming cellulose lacquer on at least one surface. The antistatic agents used are water-soluble, relatively complex compounds of sulfonated polystyrene and cycloaliphatic amine salt of an alcohol sulfate in which at least one alicyclic radical having at least 5 carbon atoms is bonded to the amine nitrogen atom. The alcohol radical of the alcohol sulfate contains at least five carbon atoms, and the compound comprises a total of at least twelve carbon atoms. Such a layer exhibits disadvantageous sensitivity to humidity, and can always be dissolved from the surface by means of water.
Further, German Offenlegungsschrift No. 2,342,601 describes a matt film material which accepts ink symbols and which contains a first coating of a matt, film-forming cellulose lacquer composition on at least one surface, and is provided with a second coating which consists of an alkyl monoester of polyalkyl vinyl ether maleic acid). Such a coating must be applied from a solution containing an organic solvent. Also, the top layers are relatively thick, so that they tend to become tacky at an elevated temperature. Also, there is no antistatic finish whatsoever.
Furthermore, a transparent drawing material is disclosed in British Pat. No. 1,231,407. The material has a base of polyester film and a drawing layer which contains silica and/or aluminum silicate as pigment and a hardened mixture of polyvinyl alcohol, a urea/ or melamine/formaldehyde precondensate and an acrylate copolymer as binder. While the material maintains a good ink line width, it has unsatisfactory wetting with ink, and lacks an antistatic finish, such that the material fails to meet the highest demands.
Finally, British Pat. No. 1,500,134 describes a polyester film which can be printed and marked with ink and which has a very thin layer, applied during the production of the film, of polyvinyl alcohol or of polyvinyl acetate which has been hydrolyzed up to 72 mole per cent, and of a second component which is a wax or a homopolymer or copolymer of acrylic or methacrylic acid, or of their esters, or a homopolymer or copolymer of vinyl acetate. Such a film, unlike a drafting film which, as a rule, can also be marked with pencil, must have a higher coating weight for the purpose of line correction. However, if such a layer is applied in drawing layer thickness, layers are obtained which are highly sensitive to moisture and heat and which are technically unacceptable. In addition, the disclosed layer lacks an antistatic finish.
A further disclosure in German Offenlegungsschrift No. 2,513,422 describes a coated plastic film such as a polyester film which, if appropriate, has applied on top of a substrate layer of acrylic or methacrylic acid copolymer, a layer which consists of an unhydrolyzed or partially hydrolyzed polymer or copolymer of vinyl acetate and a resinous component which is suitable for intramolecular cross-linking. The vinyl acetate copolymer contains 50 mole per cent of vinyl acetate or more, and can contain a very wide variety of compounds as comonomers, such as dialkyl maleate, 2-ethylhexyl acrylate, ethylene, vinyl chloride or a vinyl ester of versatic acid. Such a coated film is said to also be suitable for preparing drafting films provided appropriate additives are incorporated therein. However, no information is provided about a relevant composition.
It is true that the incorporation of finely divided delusterants is likely to modify the film material in such a way that it can be marked, to a certain extent, with pencils, but additives suitable for writing with inks require considerable experimental effort with respect to compatibility and adjustment of the substances relative to one another and with respect to required properties such as the ability of inks to wet the surface in a suitable way, adhesion to the surfaces and correctability of symbols, and the like. This is particularly important with regard to a possible addition of antistatic agents, which, as is known from experience, impair ink-writing properties, on occasion to such an extent that the inks no longer wet.